State of the art devices for measuring distances by electro-optical means are mainly based on three measurement principles:                Phase measurement        Level-based time of flight measurement        Sampling time of flight measurement        
When comparing those three methods, in general one can say that phase measurement provides the highest accuracy, while level-based time of flight measurement provides the fastest results and sampling time of flight measurement has the advantage of the best sensitivity.
By use of the phase measurement it is possible to evaluate the distance based on weak optical signals reflected back from the target of measurement, e.g. on signals within the range of few pico-watts only. On the other hand those systems have the disadvantage of a quiet high loss of signal information by the heterodyne or homodyne frequency mixing that is used in those devices, resulting in quite long measurement time. A further disadvantage is related to the difficulties of handling the often occurring multiple reflections of real-life measurement.
A level-based runtime-measurement is only capable of recognizing reflected signals with amplitude-levels above a certain threshold. Therefore low reflecting or far away targets can not be measured by such a system, as the needed high power light sources such as lasers are technically complex and also expensive. Further, eye-safety regulations are another limiting factor for the power of the laser that can be used.
One of the first distance measurement devices based on the sampling time of flight measurement was described in CH670895. An important aspect therein was to be able to measure distances over long ranges with the low optical power available when ensuring eye-safety. By the sampling method the sensitivity of the receiving electronics could be improved and also a much better signal to noise ratio (SNR) could be achieved. The accuracy of the distance measurement is not in the focus of the teaching therein. As the analog to digital converter (ADC) has only a resolution of four bits the quantification error introduced thereby is likely to limit the accuracy of the distance measurement achievable. Analogue to digital converters (ADC) usually incorporate also the sample/hold unit which is one of the 1st steps of the signal digitalisation process.
Also the document DE 36 20 226 suffers from similar drawbacks.
The publication PCT/EP2007/006226 discloses a distance measurement device utilizing the direct sampling of the received signal. Therein the distance is determined by two different approaches in parallel to allowing a wide dynamic range of the input signal and providing reliable results over a wide range of input power.
The international application PCT/EP2008/009010 discloses a device to measure distances with a moving measurement beam, whereby acquisition time and dynamics of the distance measurement are important factors. Such a device can e.g. be used for rotational scanners, profilers or laser-projectors at construction sites, enabling the evaluation of distance information along points of a track which is projected onto a surface. A time of flight measurement is used therein, but there is no information about the accuracy of the distance measurement or any teaching that there is any improvement therein.
Distance measurement devices based on the sampling runtime-measurement are characterized by directly sampling an electronically amplified signal of a reflected pulse of light received by an optoelectronic device such as a photodiode by a quantification of the signal in amplitude and time. Due to that fact, those devices are also referred to as WFD for “Wave Form Digitizer”, as the distance-evaluation is based on a digital representation of the form of the received signal.
A typical device for a distance measurement according to the WFD-principle comprises at least:                A transmitter for optical radiation, usually light-pulses emitted by a laser-diode, onto a target placed in the distance to be measured.        A receiver for those parts of the transmitted optical radiation that are scattered back from the target to the device. The resulting electrical signal is amplified by some low noise amplifier and fed into a sampling means such as an analog to digital converter (ADC) for further digital evaluation, e.g. in a FPGA, ASIC, uC, uP, DSP or such.        
In preferred embodiments, part of the emitted light is also routed on a reference path of known length and then fed directly to the receiver. The reference path can be completely device-internal as well as being at least partially outside of the device e.g. by some reference target fixed to the device somewhere inside or outside of its enclosure. As known in the art, the light pulse that travelled the reference path can be used as a reference pulse for distance evaluation and/or it can also be used for calibration of the signal-amplitudes. As this pulse takes the same signal-path except to the target-distance, environmental influences and nonlinearities of the electronics and optics can be efficiently calibrated out by this.
The digitalized shape of signal-pulses is used to determine the distance. As the device can sample a repeatedly emitted signal more than once and accumulate its digital representation in a correct alignment, it is possible to improve the signal to noise ratio by the square root of the number of accumulations and therefore devices based on the sampling runtime-measurement are able to also work with reflections of low signal strength, e.g. by poorly reflecting or far away targets. By a variation of the number of accumulations it is possible to swap the accuracy of measurement against the time needed for the measurement depending on the needs of the measurement task actually performed.
A WFD can further achieve a good signal-to-noise-ratio (SNR) because of the fact that noise is only evaluated during the short periods of time when also a pulse is present, while during the rest of the time the noise is blanked out. Thereby, the SNR is reduced by the square-root of duty cycle of the optical measurement signal. A low duty cycle also brings advantages concerning eye safety, as described further below.
While the reproducibility of the distance measurement by a state of the art WFD is quite high, the accuracy of the absolute distance is lower than the one achievable by phase-measurement (e.g. often even more than 3 mm). Therefore such a measurement can not be used for high precision rangefinders or geodetic equipment such as theodolites or 3D-Scanners as those devices commonly require a better absolute accuracy.
The distance information is evaluated by determining the travelling time of light pulses sent out by the transmitter and received by the receiver. In a WFD this is done according to the digitalized pulse information of the waveform-sampler (ADC) with appropriate high sampling rate of several 100 Mhz. In a first step this can be done by just recognizing the presence of the pulses, whereupon the travelling time of the light can be estimated within one or a few sampling periods. By this, a first rough distance information with a low resolution is achievable.
The rough resolution has an accuracy dependent on the sampling frequency fs of the ADC, resulting in a time-uncertainty of Ts=1/fs:
                                          2            ·            TOF                    Ts                =                              n            ⁢                                                  ⁢                          1              ·              NR                        ⁢                                                  ⁢            1                    +                      nf            ⁢                                                  ⁢            1                                              (        1        )            with TOF representing the “Time of Flight”, which is used by the laser pulse to travel forth or back between target and measurement device.
The measurement value nf1 denotes the number of sampling intervals between the start trigger and a characteristic signature of the received pulse of the sampled waveform. The symbol NR1 represents the number of samples in-between two pulses sent by the transmitter. Therefore NR1=1/(Ts*frep1), wherein frep1 is the pulse-rate of the transmitted optical radiation and n1 is the number of pulses travelling in-between the measuring device and the target at the same time.
For short distances n1 evaluates to zero, but if the pulse-repetition-time of the laser is less than the travelling time of the pulses for twice the distance to be measured, more than one pulses are on their way traveling the measuring distance at the same time.
For example, one of the methods, as known from the phase-measuring devices, that allows a determination of this number of pulses n1 can also be applied to the sampling distance-meter. The example further described for illustration is based on using a second emission rate for transmitting the pulses frep2, whereby a second number of sampling intervals nf2 can be evaluated.
A solution for the ambiguity of the distance can then be evaluated according to the formula:
                                          2            ·            TOF                    Ts                =                                            nf              ⁢                                                          ⁢                              1                ·                NR                            ⁢                                                          ⁢              2                        -                          nf              ⁢                                                          ⁢                              2                ·                NR                            ⁢                                                          ⁢              1                                                          NR              ⁢                                                          ⁢              2                        -                          NR              ⁢                                                          ⁢              1                                                          (        2        )            and the number of pulses n1 at a sending frequency of frep1 evaluates to:
                              n          ⁢                                          ⁢                      1            ·            NR                    ⁢                                          ⁢          1                =                  round          ⁡                      (                                                                                nf                    ⁢                                                                                  ⁢                                          1                      ·                      NR                                        ⁢                                                                                  ⁢                    2                                    -                                      nf                    ⁢                                                                                  ⁢                                          2                      ·                      NR                                        ⁢                                                                                  ⁢                    1                                                                                        NR                    ⁢                                                                                  ⁢                    2                                    -                                      NR                    ⁢                                                                                  ⁢                    1                                                              -                              nf                ⁢                                                                  ⁢                1                                      )                                              (        3        )            
The function “round( )”, thereby describes the operation of rounding up to the next integer. If n1 is introduced into formula (1) a robust, rough estimation of the distance is accomplished.
To further improve the time- and distance-resolution, certain algorithms—one of those exemplarily described in detail further below—can be used to calculate a sub-sampling-resolution of the signal and get an extremely more precise time-information, resulting also in a distance measurement with a highly accurate resolution in comparison to the sampling rate of the digitalisation.
To achieve an accuracy of the distance in-between the device and the target of e.g. 0.2 mm, a time-resolution of 1.3 ps is needed. Therefore, the ADC would require a sampling frequency of 1/1.3 ps=750*109 samples per second (750 GS/s). Such sampling rates are beyond of the state of art converters, as nowadays sampling rates of about 100 MS/s to 6 GS/s are common for such devices, wherein MS stands for Mega (106) and GS stands for Giga (109) samples). The prices of these devices rise dramatically as sampling speed increases.
To achieve a time resolution of picoseconds for at least one time related parameter of the pulse (e.g. the phase-information) is extracted from the measured signal by calculation. Due to the comparably low sampling rate, the digital data failed so far to represent all information of the received pulse. A complete direct reconstruction of the signal from the digital data is not possible, as the Nyquist-Shannon sampling theorem is not fulfilled. Therefore, many other methods have been developed for signal interpolation in state of the art devices as discussed before.
As known from the state of the art devices, as e.g. described in WO 2009/129552, a lookup-table or lookup-function containing a, preferably monotone, relation of the time related parameter and the sub-sampling time can be used for this purpose. A quite sophisticated task thereby is to generate such lookup-tables or formulas e.g. by system identification or by measurement of reference values in-between the normal sampling intervals, e.g. by shifting the sampling-time in sub-sample steps less than the sampling period.
Another equivalent approach for sub-sampling is also to shift the transmitted signal in time by such sub-sample steps, which often is easier to achieve with the required accuracy. Also this allows measuring the shape of the signal in-between the normal sampling times to get values with sub-sampling time resolution that can be used for generation of the mentioned lookup-tables or -functions.
The fact that the relation between the qualifying parameter and the sub-sampling time is dependent on lots of factors such as temperature, amplitude of the signal, clipping and nonlinearities of the receiver or the amplifier, etc. is a big challenge in such an identification task.
For the evaluation of the fine time resolution for the distance during measurement, there are also many different methods known. The interpolation of the sub-sampling time displacement of the pulses can be achieved e.g. by a cross correlation of two pulses. The disadvantage of this method is that its execution requires serious calculation effort and therefore it is quite slow. Furthermore the results are not unbiased and can comprise offsets.
Other known examples of such methods are interpolations based on one or more known features or characteristic signatures of the pulse shape based on a combination of lookup-tables and calculations. Such a feature of the pulse can e.g. be its centre of gravity, a zero crossing, its turning point of rising edge or an evaluation at different fixed or amplitude-proportional trigger levels.
A big problem with lookup-tables, as discussed above, is that they are only valid for a predefined set of pulse shapes often denoted as reference or calibration pulse. If the actual shape changes—e.g. with a rise of the temperature of the laser, by variation of the supply voltages, by target inclination to the line of sight or by aging—the values of the tables are no longer appropriate and this can lead to serious errors in the resulting absolute distance.
Especially the acquisition of the accurate values for lookup-tables, also called system identification, can be a quite difficult task.
To determine the calibration-tables during production of the device or during its usage in the field also needs lots of time and calculation effort especially as those tables are dependent on the environmental conditions, such as temperature.
A big disadvantage of lookup-tables and related correction algorithms is the fact that the slightest change of the shape of the pulse can lead to an inaccurate relation between the evaluated pulse-time and the true distance. By this approach it is hardly possible to achieve high accuracy and even more difficult to guarantee such. Some of those errors can e.g. be noticed as systematic deviations of the distance, occurring with twice the sampling-rate of the ADC, observed as a period of 3*108 m/(2*ADC_sampling_rate) in distance.
The article “Laser short-range detection system using digital processing” by LI Ping et al, from the International Symposium on Photoelectronic Detection and Imaging 2007, published in SPIE Vol. 6622, presents a modularized, FPGA and DSP based short-range detection system using a not further specified real-time processing. The system comprises an ADC preceded by a simple three stage filter of 6th order for filtering high frequency noise to improve the signal to noise ratio and prevent distortion of signal and also aliasing. This filter has ripples beyond the 3 dB point and an effective attenuation of less than −55 dB and its corner frequency is below ⅛ of the sampling rate.
The document US 2008/304043 discloses a heterodyne mixing of the received signal to lower frequencies, whereby a low speed ADC can be used. The requirements on the filtering after the mixing are also comparably low, as the frequency shift introduced by the mixing is rather big, for example a factor of 128 in the embodiment of this document.
Other known systems, such as e.g. described in WO 2005/008271 A2 avoid high frequency sampling and high frequency, high order filtering by the usage of an equivalent time sampling technology to facilitate an economical analog to digital conversion process with low sampling rates. On of the drawbacks in those solution is that many pulses have to be processed by the equivalent time sampling to bring results.